SAN FRANCISCO
— For a bird on the edge of extinction, the plan hardly seemed extreme. There were no construction projects to be halted, no parcels of prime real estate to be set aside, no logging rights to be curtailed.

To help save the Xantus murrelet, which clings to its existence on the sheer rock faces and coves of a few volcanic islands along the Pacific coast, all that needed to be done was exterminate a colony of rats that weren't native to the islands anyway.

Instead, as the National Park Service aims to go ahead with the plan as early as tomorrow, it finds venomous opposition, but from an unusual source: environmentalists.

From California's coastal islets to the Grand Canyon, such efforts to eradicate "exotic" animals  brought by humans, knowingly or ignorantly, to new habitats  are exposing a growing rift in the environmental world.

On one side are the traditional conservationists, such as the Audubon Society, which fear that exotic species like the black rats of Anacapa Island will destroy an ecosystem carefully crafted over millenniums. On the other are animal-rights activists, who stand unequivocally for the rights of each rodent and raptor.

The split has been building for a decade as concern mounts over the impact of exotic species from snakehead fish in Maryland to tree-eating beetles in New York. Now, on this spit of black rock and shrub south of Santa Barbara, these two schools of environmentalism are facing one of their most pitched battles yet.

To Gavin Shire of the American Bird Conservatory, the need for action could not be clearer. As few as 5,000 of the black-backed, white-breasted seabirds exist, all on nine islands that stretch from Mexico to California's Channel Islands.

Park officials say they have found rat droppings near scavenged murrelet eggs and telltale rat bite marks on eggs.

The program, in fact, is already half done. Last autumn, the Park Service dropped poison pellets on half of Anacapa. Since then, no rats have been found there, while one murrelet egg has appeared in a previously barren cave.

"Just having that one intact egg is giving us a lot of hope," says the park's Kate Faulkner.

Tough choices

Yet animal-rights activists, which have tried to stop the program in court, count the cost differently.

They suggest that the only comprehensive environmental review of Anacapa Island shows that the rats do not eat murrelets. This effort, then  like others across the country  is at best misguided and at worst a kind of animal racism. And the very notion of exotic species, they say, merely provides a rationale for killing unwanted animals.

"Once species integrate into the ecosystem, ripping them out can hurt even endangered animals," says Scarlet Newton of the Channel Islands Animal Protection Association. "They want to 'Disneyfy' the island."

Signs of a broader backlash are apparent. Protesters in 1997 chained themselves to a buoy in California's Lake Davis when authorities planned to poison the lake to get rid of nonnative northern pike. This month in Arizona, activists are opposing a plan to temporarily flood parts of the Grand Canyon to help humpback chub fish compete with nonnative trout.

On Anacapa, critics say the project has been a catastrophe. Sea kayakers and boats disturbing nesting places have primarily caused the murrelet's problems, they say. Meanwhile, the poisoning has killed at least 49 birds unintentionally and decimated the Anacapa deer mouse  also a rare native species.

It's all just about tough choices, conservationists counter. The mouse will recover, they say, through a catch-and-release program, and the second poison drop  which could begin tomorrow  is necessary.

Says Shire: "We believe the justification comes in the saving of an endangered species."